Methods for Improving the Recovery of Metal Leaching Agents

ABSTRACT

Processes for metal leaching/solvent extraction are described which comprise: (a) providing a first aqueous leach pulp which comprises a mixture of leached solids and an aqueous leach solution comprising a metal, a leaching agent and water; (b) subjecting the first aqueous leach pulp to a first solid-liquid separation to provide a first clarified aqueous leach solution and a second aqueous leach pulp, wherein the second aqueous leach pulp comprises the leached solids at a % solids level greater than the first pulp; (c) subjecting the first clarified aqueous leach solution to a first solvent extraction prior to any significant dilution, whereby a first aqueous raffinate is obtained; (d) subjecting the second aqueous leach pulp to a second solid-liquid separation with dilution via an aqueous stream to obtain a second clarified aqueous leach solution; and (e) subjecting the second clarified aqueous leach solution to a second solvent extraction whereby a second is aqueous raffinate is obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/895,794,filed Jul. 21, 2004, which claims priority, under 35 U.S.C. §119(e), ofU.S. Provisional Patent Application No. 60/491,311, filed on Jul. 30,2003, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Most metals are obtained by removing those metal values from the ores inwhich they are found in the ground. Once the ore has been mined, themetal must then be separated from the remainder of the ore. One methodto separate the metal from the ore is known as leaching. In general, thefirst step in this process is contacting the mined ore with an aqueousis solution containing a leaching agent which extracts the metal fromthe ore into solution. For example, in copper leaching operations, suchas, for example, in the agitation leaching of copper oxide ores,sulfuric acid in an aqueous solution is contacted with copper oxideminerals. During the leaching process, acid in the leach solution isconsumed and copper is dissolved thereby increasing the copper contentof the aqueous solution.

The aqueous leach solution containing the leached metal can then betreated via a known process referred to as solvent extraction whereinthe aqueous leach solution is contacted with a nonaqueous solutioncontaining a metal-specific extraction reagent. The metal-specificextraction reagent extracts the metal from the aqueous phase into thenon-aqueous phase. During the solvent extraction process for copper andcertain other metals, the leaching agent is regenerated in the aqueousphase. In the case where sulfuric acid is the leaching agent, sulfuricacid is regenerated in the aqueous phase when copper is extracted intothe organic phase by the extraction reagent. Normally, for every ton ofcopper removed from the leach solution about 1.5 tons of sulfuric acidis generated in the leach solution.

Leaching agents are often recycled back to the leaching process todissolve more metal and the more leaching agent that can be recycled theless that needs to be obtained from another source. In a standardagitation leaching process for copper, followed by solvent extraction,the leach solution is diluted to a lesser or greater extent with waterin conjunction with the solid-liquid separation process needed toprovide a clarified leach liquor and tailings. The diluted clarifiedleach solution is then transferred to one or more solvent extractionplants depending on the volume of leach solution and the capacity ofeach plant. The diluted leach solution undergoes solvent extractionwherein copper is removed from, and the sulfuric acid concentration isincreased in, the aqueous phase. A portion of this copper-depleted,acid-containing aqueous phase, now called the raffinate, is thenrecycled back to the leaching process. The other portion is recycledback to the front of the solid-liquid separation process where itdilutes the leach solution exiting the agitation leaching process.Depending on the acid balance across the whole process some of thisrecycled aqueous phase may be partially neutralized.

The leach solution from an agitation leach process is normally dilutedduring the solid-liquid separation step in order to maximize the washingof the leached solids so that metal lost to the solids is minimized.During solvent extraction as the metal is extracted, acid concentrationbuilds in the aqueous phase and the reaction becomes self-limiting inequilibrium. However, because of the initial dilution to maximize metalrecovery from the leached solids, the amount of acid regenerated islower in concentration than it would have been if the leach solution hadnot been diluted in the washing of the leached solids. Unfortunately,the lower the concentration of acid in the recycled raffinate, the morefresh acid that needs to be added and this increases the cost of theoperation.

Accordingly, there is a need in the art for improved processes for metalleaching and solvent extraction, wherein the recovery of leaching agentsis improved without negatively affecting metal recovery.

BRIEF SUMMARY OF THE INVENTION

The present invention relates, in general, to metal leaching operationsand methods of improving the recovery of leaching agents from solventextraction operations.

It has been surprisingly discovered that by splitting an aqueous leachsolution into two or more portions and subjecting at least one portionto solvent extraction prior to any significant dilution and alsosubjecting at least one other portion to solvent extraction afterdilution, (also referred to herein as a “split circuit”), that good, andeven optimum, metal extraction can be achieved while significantlyimproving the recovery of the leaching agent.

One embodiment of the present invention includes processes whichcomprise: (a) providing a first aqueous leach pulp which comprises amixture of leached solids and an aqueous leach solution comprising ametal, a leaching agent and water; (b) subjecting the first aqueousleach pulp to a first solid-liquid separation to provide a firstclarified aqueous leach solution and a second aqueous leach pulp,wherein the second aqueous leach pulp comprises the leached solids at a% solids level greater than the first pulp; (c) subjecting the firstclarified aqueous leach solution to a first solvent extraction prior toany significant dilution, whereby a first aqueous raffinate is obtained;(d) subjecting the second aqueous leach pulp to a second solid-liquidseparation with significant dilution via an aqueous stream to obtain asecond clarified aqueous leach solution; and (e) subjecting the secondclarified aqueous leach solution to a second solvent extraction wherebya second aqueous raffinate is obtained.

In many of the preferred embodiments of the present invention, the metalcomprises copper. Also, in many preferred embodiments of the presentinvention, the leaching agent comprises sulfuric acid. In more preferredembodiments of the present invention, the metal comprises copper and theleaching agent comprises sulfuric acid.

Another embodiment of the present invention includes processes whichcomprise: (a) providing a first aqueous leach pulp obtained from anagitation leaching process, wherein the first aqueous leach pulpcomprises a mixture of leached solids and an aqueous leach solutioncomprising copper, sulfuric acid and water; (b) subjecting the firstaqueous leach pulp to a first solid-liquid separation to provide a firstclarified aqueous leach solution and a second aqueous leach pulp,wherein the second aqueous leach pulp comprises the leached solids at a% solids level greater than the first pulp; (c) subjecting the firstclarified aqueous leach solution to a first solvent extraction prior toany significant dilution, whereby a first aqueous raffinate is obtained;(d) subjecting the second aqueous leach pulp to a second solid-liquidseparation with significant dilution via an aqueous stream to obtain asecond clarified aqueous leach solution, wherein the concentration ofthe metal in the first clarified aqueous leach solution is greater thanthe concentration of the metal in the second clarified aqueous leachsolution; (e) subjecting the diluted second portion to a second solventextraction whereby a second aqueous raffinate is obtained; wherein theaqueous stream for diluting the second aqueous leach pulp comprises atleast a portion of the second aqueous raffinate; and (f) recycling atleast a portion of the first aqueous raffinate and at least a portion ofthe second aqueous raffinate to the leaching process.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a process flow diagram representing a standardleaching/solvent extraction operation wherein all of the aqueous leachsolution is treated in the same manner.

FIG. 2 is a process flow diagram representing a preferred embodiment ofthe present invention wherein an aqueous leach solution is divided intotwo portions and subjected to solvent extraction under two differentsets of conditions.

DETAILED DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients or reaction conditions usedherein are to be understood as modified in all instances by the term“about”.

Aqueous leach pulps from agitation leaching operations comprise amixture of leached solids (i.e., ore residues) and an aqueous leachsolution. Aqueous leach solutions comprise water, a leaching agent and ametal. Aqueous leach solutions can additionally contain other metals,impurities, and residual leached solids. Aqueous leach pulps areobtained from the treatment of ground or milled ores with an aqueoussolution of a leaching agent. The aqueous leach pulp then flows or iscarried to further processing and solvent extraction. The manner inwhich the aqueous leach pulp, or any other solution, stream or raffinateis conveyed during the processes according to the present invention isinconsequential. In general, pulps, solutions, streams and raffinatesmay be conveyed by pipe or any other natural or man-made conduit.

In accordance with the present invention, a first aqueous leach pulp issubjected to a solid-liquid separation to remove at least some of theleached solids which are present therein. The first aqueous leach pulpis divided into two or more portions by subjecting the aqueous leachpulp to a solid-liquid separation, such as a decantation-clarifier orfiltering, to provide a first portion or first clarified aqueous leachsolution and a second portion or second aqueous leach pulp, wherein thesecond pulp contains leached solids at a higher % solids level than thefirst aqueous leach pulp. Essentially, the solid-liquid separation isused to divide the solution into the two portions for separate solventextraction. The first portion is a clarified or partially clarifiedleach solution while the second portion is a combination of leach liquorand leached solids at a higher solids content than the first aqueousleach pulp. The clarified or partially clarified leach solution advancesto solvent extraction while the second portion advances to a countercurrent decantation wash circuit or to another type of solid-liquidseparation that includes some washing of the solids.

In general, each solid-liquid separation can be carried out in any knownmanner. Any method for separating solids from liquids can be employed.The manner in which solid-liquid extraction is carried out is notcritical. For example, solids can be separated from liquids by methodsincluding, but not limited to, decantation and/or filtration.Decantation is preferred.

The processes according to the present invention can be used in anymetal recovery operation which employs an aqueous agitation leachingoperation where the leaching agent is regenerated in the solventextraction process. Thus, the processes according to the presentinvention can be applicable to any metal leached by an aqueous solution.Such metals include the transition metals. The processes according tothe present invention are preferably employed in the leaching of metalswhich occur naturally as oxide and/or sulfide ores. The processesaccording to the present invention are most preferably used in theleaching of divalent metal ores. Such metals include copper, zinc,cobalt and nickel. The processes according to the present invention aremost preferably used in the leaching of copper.

The aqueous leach solutions treated in the processes according to thepresent invention contain a leaching agent which is capable of leachingthe metal from the ore with which they are previously contacted. Theprocesses according to the present invention are applicable to leachingoperations wherein an aqueous leaching agent is employed. In certainpreferred embodiments of the present invention the leaching agentcomprises sulfuric acid. In those preferred embodiments of the presentinvention where the metal comprises copper, it is further preferred thatsulfuric acid be used as the leaching agent. Other leaching agents whichcan be used in processes according to the present invention include, butare not limited to acids such as hydrochloric acid, nitric acid, organicacids and combinations thereof, and basic substances such as ammonia.Essentially, any leaching agent which is water-miscible, capable ofleaching the metal from the ore and which produces a water-solubleleaching agent-metal compound can be used.

In the processes according to the present invention, the first aqueousleach pulp is divided into at least two portions prior to any solventextraction, a first clarified leach solution and a second aqueous leachpulp containing a greater % solids than the first aqueous leach pulp.Division of the first aqueous leach pulp can be accomplished via anyknown process of splitting a leach pulp into two or more separatestreams or volumes. In general, the first aqueous leach pulp is dividedinto two portions. The first clarified leach solution is subjected tosolvent extraction prior to any significant dilution and the secondaqueous leach pulp is taken through a dilution wash circuit to produce adiluted second clarified leach solution which is then subjected tosolvent extraction. However, the clarified aqueous leach solutions canbe divided into more than two streams, for example, where multiplecircuits are running in parallel. For example, the first clarified leachsolution can be further divided into two portions which proceed to twosolvent extraction plants without any significant dilution while thesecond leach pulp undergoes a solid-liquid separation to give one streamof a second clarified leach solution which then proceeds to one solventextraction plant or vice versa. In a similar manner the first clarifiedleach solution can be further divided into two portions which proceed totwo separate solvent extraction plants and the second clarified aqueousleach solution could also be divided into two separate streams whichproceed to two separate solvent extraction plants. The way the leachsolutions are divided will depend on many factors such as the metalcontent of the original leach solution, the design of the solventextraction plant, the response of the leach solids to solid-liquidseparation and the total flow of leach solution to be treated. Theimportant feature of the division of the leach solution is to take asmuch of the metal that is leached to the first solvent extractionplant(s) so as to maximize the regeneration of the leaching agent. Addeddivision of the leach solution can occur where volume and capacityrequire.

The division of the aqueous leach solution in accordance with theprocesses of the present invention can be done evenly or such that oneportion contains a greater volume than the other. In certain preferredembodiments of the present invention, the dividing of the aqueous leachsolution is carried out such that the volume of the leach solutionpresent in the portion which is subjected to solvent extraction prior toany significant dilution is greater than the volume of leach solutionpresent in the portion which is diluted prior to solvent extraction.

As used herein, the term “significant dilution” refers to the additionof a measurable amount of water or other aqueous solution to an aqueousleach solution. Accordingly, significant dilution of the second aqueousleach pulp refers to the addition of water or other aqueous solution tothe second aqueous leach pulp in an amount such that the concentrationof metal in the first clarified aqueous leach solution is greater thanthe concentration of the metal in the second clarified aqueous leachsolution. In preferred embodiments of the present invention, theconcentration of metal in the first clarified aqueous leach solution isat least 10% greater than the concentration of the metal in the secondclarified aqueous leach solution. In increasingly more preferredembodiments of the present invention, the concentration of metal in thefirst clarified aqueous leach solution is at least 20% greater, at least30% greater, at least 40% greater, at least 50% greater, at least 60%greater, at least 70% greater, at least 80% greater, at least 90%greater, at least 100% greater, at least 200% greater, at least 300%greater, at least 400% greater, at least 500% greater, or even higherthan the concentration of the metal in the second clarified aqueousleach solution. In the most preferred embodiments of the presentinvention, the first clarified aqueous leach solution is subjected tosolvent extraction without any dilution. However, it is to be understoodthat water or other aqueous solution can be added to the first clarifiedaqueous leach solution prior to the first solvent extraction, but onlyin such amounts that the concentration of metal in the first clarifiedaqueous leach solution prior to solvent extraction remains greater thanthe concentration of the metal in the second clarified aqueous leachsolution. However, as increasing dilution of the first clarified aqueousleach solution decreases leaching agent recovery, less dilution ispreferred.

Solvent extraction in accordance with the processes of the presentinvention can be carried out in any known manner wherein aqueous leachsolution is contacted with an organic phase containing a metal-specificextraction reagent. Each extraction performed in accordance with thepresent invention can be carried out by mixing the organic phase and theaqueous leach agent and allowing the two phases to settle. Thismixing-settling can be carried out in multiple series of mixing-settlingtanks with countercurrent flow of the aqueous and non-aqueous phases.

The aqueous phase resulting from a solvent extraction operation isreferred to as a raffinate. In the processes according to the presentinvention, the first portion of the aqueous leach solution is subjectedto solvent extraction prior to any significant dilution and a firstaqueous raffinate is obtained. In the processes according to the presentinvention, the second portion of the aqueous leach solution is dilutedwith an aqueous stream and then subjected to a separate solventextraction and a second aqueous raffinate is obtained. The firstraffinate produced in accordance with the processes of the presentinvention will generally have a leaching agent concentration which isgreater than the concentration of leaching agent present in the secondraffinate. In preferred embodiments of the present invention, the firstraffinate will have a leaching agent concentration which is is at least10% greater than the concentration of leaching agent present in thesecond raffinate. In certain increasingly more preferred embodiments ofthe present invention, the first raffinate will have a leaching agentconcentration which is at least 20% greater, 30% greater, 40% greater,50% greater, 60% greater, 70% greater, 80% greater, 90% greater, 100%greater, 200% greater, or more than the concentration of leaching agentpresent in the second raffinate.

In the processes according to the present invention, the second aqueousleach pulp is diluted prior to being subjected to solvent extraction.The second aqueous leach pulp is diluted with an aqueous stream. Theaqueous stream for diluting the second aqueous leach pulp can comprisefresh water introduced into the process, at least a portion of theaqueous raffinate from another solvent extraction plant, at least aportion of the second aqueous raffinate, or a combination thereof. Incertain preferred embodiments of the present invention, the secondaqueous leach pulp is diluted with at least a portion of the secondaqueous raffinate. Where the leaching agent comprises an acid, thesecond aqueous raffinate can be at least partly neutralized prior to itsuse for diluting the second aqueous leach pulp. Neutralization can beaccomplished via the addition of any basic substance. In thoseembodiments wherein the leaching agent comprises sulfuric acid, lime ispreferred for neutralization. Neutralization need not be complete. Asuitable pH range for the partly neutralized second aqueous raffinateprior to its use for dilution is any pH up to about 8.

In the processes according to the present invention, a portion of thesecond aqueous raffinate may be bled from the circuit to maintain waterbalance. Additionally, in certain preferred embodiments of the presentinvention, at least a portion of the first aqueous raffinate is recycledto a leaching operation where the leaching agent contained therein isemployed to leach more metal from ore. In more preferred embodiments, atleast a portion of the first aqueous raffinate is recycled to the sameleaching operation from which the aqueous leach solution was obtained.In certain other preferred embodiments of the present invention, atleast a portion of the second aqueous raffinate is recycled to aleaching operation where the leaching agent contained therein isemployed to leach more metal from ore. In more preferred embodiments, atleast a portion of the second aqueous raffinate is recycled to the sameleaching operation from which the aqueous leach solution was obtained.In even more preferred embodiments of the present invention at least aportion of both the first and the second aqueous raffinates are recycledto a leaching operation where the leaching agent contained therein isemployed to leach more metal from ore. In still yet more preferredembodiments, at least a portion of both the first and the second aqueousraffinates are recycled to the same leaching operation from which theaqueous leach solution was obtained.

FIG. 1 depicts a process flow diagram of a standard, prior art agitationleach process for copper followed by solvent extraction. The leach pulpexiting leaching (LEACH), about 190 cubic meters/hour, is mixed incounter current decantation (S/L SEPARATION) with about 630 cubicmeters/hour of recycled raffinate from the dual copper solventextraction plants (SX 1 & SX 2). Neutralization of the recycledraffinate is optional. In this way the copper concentration is dilutedfrom about 24 g/l Cu to about 6.0 g/l Cu prior to being fed to thesolvent extraction circuit. The solvent extraction circuit consists of 2separate plants or trains labeled SX 1 and SX 2, respectively, with eachplant treating about 400 cubic meters/hour of aqueous solution flow. Theraffinate exiting the solvent extraction plants are combined and then aportion of this solution (about 160 cubic meters/hour) is recycled tothe leaching vessel where the acid in the leach solution is used todissolve the copper. A second portion of this solution is recycled tothe counter current solid-liquid separation operation where it is usedto wash the leach solution from the leached solids so as to minimizemetal losses to the leached solids that are eventually disposed totailings. A small portion of fresh water may be added to the overallleach/wash system or a small portion of aqueous solution may be bledfrom the overall leach/wash system to maintain a water balance.

FIG. 2 depicts a process flow diagram of a leaching process for copperfollowed by solvent extraction according to a preferred embodiment ofthe present invention. The aqueous leach pulp exiting the leach vessel(LEACH), about 190 cubic meters/hour, passes through an initialsolids-liquid separation (S/L SEPARATION). Then about 120 cubicmeters/hour of this solution containing about 26.2 g/l Cu is takendirectly to solvent extraction (SX 1) where the copper is extracted andsulfuric acid is produced. SX 1 will reasonably produce a raffinatecontaining about 4 g/l Cu and 35 g/l acid. This solution is thenrecycled back to leaching. The aqueous portion of the leach solutionremaining in the leach solution pulp that has exited the initialsolid-liquid separation that does not proceed to SX 1 (about 70 meterscubed/hour) is taken to a counter current decantation (CCD) where it ismixed with about 350 cubic meters/hour of raffinate from SX 2 that hasbeen optionally, partially neutralized. Then about 400 meters cubed ofleach solution from the CCD circuit containing 4.94 g/l Cu is taken toSX 2 to give a raffinate containing 0.4 g/l Cu and 8 g/l acid. A smallportion of raffinate from SX 2 may be bled from the circuit to maintainwater balance. Additionally about 40 meters cubed/hour of raffinate fromSX 2 is returned to the leaching vessel.

One advantage of the process according to the present invention is thatmuch more acid is returned to leaching than with the standard process.For example, by comparing the standard process depicted in FIG. 1 withthe preferred embodiment of the present invention depicted in FIG. 2, itcan be seen that in the standard process, 160 meters cubed/hour ofraffinate containing about 9.5 g/l sulfuric acid is returned to theleaching vessel bringing with it about 1.52 metric tons of acid perhour. In the process according to a preferred embodiment of theinvention, 120 meters cubed/hour of raffinate from SX 1 and 40 meterscubed/hour of raffinate from SX 2 are returned to the leaching vesselbringing a total of about 4.54 tons of acid back to leaching. Thisrepresents a savings of about 3.02 tons of acid/hour or about 72.5 tonsof acid/day.

A second advantage of the process according to the present invention isrealized in the neutralization of the recirculating raffinate ifneutralization is needed. For example, by comparing the standard processdepicted in FIG. 1 with the preferred embodiment of the presentinvention depicted in FIG. 2, it can be seen that in the standardprocess about 630 meters cubed/hour of solution containing about 9.5 g/lacid is neutralized while in the process according to a preferredembodiment of the present invention, about 350 cubic meters/hour ofsolution containing about 8 g/l acid is neutralized. This results in theneed for significantly less neutralization agent for the practice ofthis invention over standard practice.

A third advantage of the process according to the present invention isthat the bleed with the process according to the invention may in factcontain less metal than the bleed with the normal configuration. FIG. 1shows that the bleed for the normal circuit will contain about 0.5 g/lCu and 9.5 g/l H₂SO₄ while the bleed in the process according to thepreferred embodiment of the invention depicted therein will contain onlyabout 0.4 g/l Cu and 8 g/l H₂SO₄. In fact because the feed to SX 1 andSX 2 in the standard process has about 6.05 g/l Cu while the feed to SX2 in the preferred embodiment of the inventive process depicted in FIG.2 has about 4.94 g/l it is readily apparent to one skilled in the artthat SX 2 in the process according to the invention will produce araffinate lower in copper than either SX 1 or SX 2 in the standardprocess.

A fourth advantage of the split circuit design pertains to coppersolvent extraction plants where a component of value in the bleed isrecovered, for example cobalt. In most cases the bleed must beneutralized prior to cobalt recovery. Neutralization with a soluble basesuch as caustic or ammonia is very expensive therefore the lower theacid content of the bleed stream the lower the amount of base needed forneutralization. Furthermore the use of a solution of caustic forneutralization adds water to the bleed stream thereby diluting thevaluable cobalt stream. Alternatively neutralization can take place withlime or limestone which is a less costly base. In this case a lesseramount of acid in the bleed stream requires less lime or limestone forneutralization and in the process a lesser amount of gypsum precipitateis produced. Gypsum must be removed from the system and all the solutioncontaining the valuable metal must be recovered. A lesser amount ofgypsum allows the use of smaller equipment for the solid-liquidseparation. When finely divided solids are separated from a liquid thesolids will always contain some of the liquid. In the case underdiscussion the lesser amount of gypsum will contain a lower volume ofthe neutralized bleed stream which contains the valuable secondcomponent, for example cobalt. Thus the ultimate recovery of thevaluable component in the bleed stream is higher when using the processaccording to the invention.

The present invention will now be illustrated in more detail byreference to the following specific, non-limiting examples.

COMPARATIVE EXAMPLE A & EXAMPLE B

In Comparative Example A, based on FIG. 1, an aqueous leach solution isobtained from a leaching operation that produces about 190 cubicmeters/hour of leach solution containing 24 g/l Cu and about 1 g/lsulfuric acid. This leach solution is mixed with a high volume ofrecycled and optionally, partially neutralized raffinate, 630 cubicmeters/hour containing 0.5 g/l Cu and about 1 g/l sulfuric acid, toproduce an aqueous solution of about 800 cubic meters/hour containingabout 6.05 g/l Cu and about 1 g/l sulfuric acid. The 800 cubicmeters/hour of solution is split into two equal streams and each streamis then fed to copper solvent extraction plant. Copper extractionisotherms followed by computer modeling show that the copper solventextraction can be expected to produce a raffinate containing about 0.5g/l Cu and about 9.5 g/l sulfuric acid. This represents a copperrecovery of 91.7% which is well within the recovery that can be expectedin a commercial copper solvent extraction plant.

In Comparative Example A, 160 cubic meters of raffinate containing 9.5g/l sulfuric acid would return to leaching carrying 1.52 metric tons ofacid per hour to leaching.

In Example B, based on FIG. 2, an aqueous leach solution is obtainedfrom a leaching operation that produces 190 cubic meters of leachsolution containing 26.2 g/l Cu and about 1.0 g/l sulfuric acid. Thisleach solution goes directly to a solid-liquid separation which occursin a clarifier using decantation. Then about 120 cubic meters of theclarified leach solution is taken to a first solvent extraction plantwhere copper is extracted and sulfuric acid is produced. Extractionisotherms and computer modeling show that a raffinate containing about 4g/l Cu and about 35.2 g/l sulfuric acid can easily be produced byadvancing 400 cubic meters of organic flow using a reagent concentrationof about 25 to 30 volume % reagent. In this case the acid returned toleaching in the 120 cubic meters of raffinate is about 4.22 metrictons/hour.

Also, in Example B, an additional 40 cubic meters of recycled aqueoussolution containing about 8.0 g/l sulfuric acid is returned to leaching.This brings an additional about 0.32 tons acid/hour to leaching. Thusthe total acid returned to leaching using a process according to thispreferred embodiment of the present invention is about 4.54 tons perhour.

A simple calculation shows that for this example the acid savings usingthe split circuit are about 4.54 metric tons/hour less 1.52 metrictons/hour=about 3.02 metric tons/hour or about 72.5 metric tonsacid/day. Acid costs vary widely from as low as US$ 15/ton to above US$150/ton depending on the location. For low cost-acid, the savings wouldbe about US$ 1088/day, while for high cost-acid the savings would beabout US$ 10,880/day or higher.

In Comparative Example A, the neutralization of the acid is carried outon a large portion of the recycled raffinate that does not proceeddirectly to leaching, but, rather is used to dilute the leach solutionprior to solvent extraction. About 630 cubic meters of raffinate flow istaken to neutralization. Additionally, a bleed of the raffinate prior toneutralization is needed to maintain water balance in the circuit andcan be as high as 20 to 25% or as low as only a few % of the flow of theleach solution exiting the leaching vessel. In Comparative Example A,there is a bleed of 10 meters cubed/hour and a raffinate stream to beneutralized of 630 cubic meters/hour. The raffinate contains about 0.5g/l Cu and about 9.5 sulfuric acid. When this raffinate is neutralizedto a pH of about 1.8 it will contain about 1 g/l sulfuric acid so thetotal acid neutralized is about 5.36 metric tons/hour (630 cubicmeters/hour×8.5 kilos acid/cubic meter).

In Example B, the total raffinate taken to neutralization is about 350meters/cubed hour containing about 8.0 g/l sulfuric acid. Uponneutralization to a pH of 1.8, the total acid neutralized is about 2.45metric tons/hour (350 cubic meters/hour×7.0 kilos acid/cubic meter). Thesavings in neutralization are about 2.91 metric tons acid per hour (5.36less 2.45). This is a significant improvement because less acid needs tobe neutralized. Less acid neutralization means that smaller equipment isneeded for neutralization and less base is needed for theneutralization. Using lime as the neutralization agent in ComparativeExample A produces more than twice the amount of precipitated gypsum asthe neutralization of acid with lime in Example B. Thus the equipmentneeded for neutralization and the equipment needed for the solid-liquidseparation after neutralization will be more than twice in size inComparative Example A than in Example B.

In Example B, further savings in neutralization are realized because theleached tailings slurry exiting the solid-liquid separation must beneutralized to a pH of about 7 to 7.5. In Comparative Example A, thewater contained in the leached tailings may contain up to about 7.5 g/lsulfuric acid. In Example B, the water contained in the leached tailingsmay contain up to about 6.5 g/l sulfuric acid.

In addition the water exiting the CCD circuit with the washed solids (40meter cubed per hour) contains 0.8 g/l Cu in the standard practice whilethe same amount of water in the practice of this invention only contains0.6 g/l Cu. Thus for the exact same amount of copper ore leached thepractice of the present invention will produce about 192 kilos of coppermore per day (40×0.2×24).

The bleed from the example under consideration is 10 cubic meters /hour. In the standard practice this bleed contains about 0.5 g/l Cuwhile the bleed in the practice of this invention only contains about0.4 g/l Cu. Thus the copper lost in the bleed for the standard practiceis about 1 kilo of copper per hour more than the copper lost in thebleed for the practice of this invention. For a small bleed thedifference in copper lost in the standard practice compared to thepractice of this invention is quite small but for a plant that has a 20%to 25% bleed the difference in copper lost can be significant.

In reference to optional neutralization of recycled raffinate it will beappreciated by those skilled in the art that the level of neutralizationis dependent on the acid consumed by the leached solids as the leachedis solids proceed through the solid-liquid separation. In some casesconsiderable acid will be consumed during the solid-liquid separationand little or no neutralization of the recycled raffinate will beneeded. In other cases only small amounts of acid may be consumed duringsolid-liquid separation and neutralization of the recycled raffinate maybe more extensive.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A process comprising: (a) providing a first aqueous leach pulp whichcomprises a mixture of leached solids and an aqueous leach solutioncomprising a metal, a leaching agent and water; (b) subjecting the firstaqueous leach pulp to a first solid-liquid separation to provide a firstclarified aqueous leach solution and a second aqueous leach pulp,wherein the second aqueous leach pulp comprises the leached solids at a% solids level greater than the first pulp; (c) subjecting the firstclarified aqueous leach solution to a first solvent extraction prior toany significant dilution, whereby a first aqueous raffinate is obtained;(d) subjecting the second aqueous leach pulp to a second solid-liquidseparation with significant dilution via an aqueous stream to obtain asecond clarified aqueous leach solution; and (e) subjecting the secondclarified aqueous leach solution to a second solvent extraction wherebya second aqueous raffinate is obtained.
 2. The process according toclaim 1, wherein said first aqueous leach pulp is obtained from anagitation leaching process, wherein said leaching agent is an acid, andsaid metal is selected from the group consisting of zinc, cobalt, andnickel.
 3. The process according to claim 1, wherein said leaching agentis ammonia, and wherein said metal is selected from the group consistingof copper, zinc, cobalt, and nickel.
 4. The process according to claim1, wherein the volume of the aqueous leach solution in the firstclarified aqueous leach solution is greater than the volume of theaqueous leach solution in the second aqueous leach pulp.
 5. The processaccording to claim 1, wherein the aqueous stream for diluting the secondaqueous leach pulp comprises the second aqueous raffinate.
 6. Theprocess according to claim 5, wherein the aqueous stream comprising thesecond aqueous raffinate is at least partly neutralized prior todilution of the second aqueous leach pulp.
 7. The process according toclaim 1, wherein the second aqueous leach pulp is subjected to thesecond solid-liquid separation prior to dilution.
 8. The processaccording to claim 1, wherein the second aqueous leach pulp is subjectedto the second solid-liquid separation simultaneously with dilution. 9.The process according to claim 8, wherein the second solid-liquidseparation comprises counter-current decantation.
 10. The processaccording to claim 1, wherein the concentration of the metal in thefirst clarified aqueous leach solution is at least 10% greater than theconcentration of the metal in the second clarified aqueous leachsolution.
 11. The process according to claim 1, wherein theconcentration of the metal in the first clarified aqueous leach solutionis at least 50% greater than the concentration of the metal in thesecond clarified aqueous leach solution.
 12. The process according toclaim 1, wherein the concentration of the metal in the first clarifiedaqueous leach solution is at least 100% greater than the concentrationof the metal in the second clarified aqueous leach solution.
 13. Theprocess according to claim 1, wherein at least a portion of the firstaqueous raffinate is recycled to a leaching process.
 14. The processaccording to claim 1, wherein the first aqueous leach pulp is obtainedfrom a leaching process and wherein at least a portion of the firstaqueous raffinate is recycled to the leaching process.
 15. The processaccording to claim 1, wherein at least a portion of the second aqueousraffinate is recycled to a leaching process.
 16. The process accordingto claim 1, wherein the first aqueous leach pulp is obtained from aleaching process and wherein at least a portion of the second aqueousraffinate is recycled to the leaching process.
 17. The process accordingto claim 1, wherein the first aqueous leach pulp is obtained from aleaching process and wherein at least a portion of the first aqueousraffinate and at least a portion of the second aqueous raffinate arerecycled to the leaching process.
 18. A process comprising: (a)providing a first aqueous leach pulp obtained from an agitation leachingprocess, wherein the first aqueous leach pulp comprises a mixture ofleached solids and an aqueous leach solution comprising an acid, water,and a metal selected from the group consisting of zinc, cobalt, andnickel; (b) subjecting the first aqueous leach pulp to a firstsolid-liquid separation to provide a first clarified aqueous leachsolution and a second aqueous leach pulp, wherein the second aqueousleach pulp comprises the leached solids at a % solids level greater thanthe first pulp; (c) subjecting the first clarified aqueous leachsolution to a first solvent extraction prior to any significantdilution, whereby a first aqueous raffinate is obtained; (d) subjectingthe second aqueous leach pulp to a second solid-liquid separation withsignificant dilution via an aqueous stream to obtain a second clarifiedaqueous leach solution, wherein the concentration of the metal in thefirst clarified aqueous leach solution is at least 10% greater than theconcentration of the metal in the second clarified aqueous leachsolution; (e) subjecting the diluted second portion to a second solventextraction whereby a second aqueous raffinate is obtained; wherein theaqueous stream for diluting the second aqueous leach pulp comprises atleast a portion of the second aqueous raffinate; and (f) recycling atleast a portion of the first aqueous raffinate and at least a portion ofthe second aqueous raffinate to the leaching process.
 19. The processaccording to claim 18, wherein the concentration of the metal in thefirst clarified aqueous leach solution is at least 50% greater than theconcentration of the metal in the second clarified aqueous leachsolution.
 20. The process according to claim 18, wherein theconcentration of the metal in the first clarified aqueous leach solutionis at least 100% greater than the concentration of the metal in thesecond clarified aqueous leach solution.
 21. A process comprising: (a)providing a first aqueous leach pulp, wherein the first aqueous leachpulp comprises a mixture of leached solids and an aqueous leach solutioncomprising ammonia, water, and a metal selected from the groupconsisting of copper, zinc, cobalt, and nickel; (b) subjecting the firstaqueous leach pulp to a first solid-liquid separation to provide a firstclarified aqueous leach solution and a second aqueous leach pulp,wherein the second aqueous leach pulp comprises the leached solids at a% solids level greater than the first pulp; (c) subjecting the firstclarified aqueous leach solution to a first solvent extraction prior toany significant dilution, whereby a first aqueous raffinate is obtained;(d) subjecting the second aqueous leach pulp to a second solid-liquidseparation with significant dilution via an aqueous stream to obtain asecond clarified aqueous leach solution, wherein the concentration ofthe metal in the first clarified aqueous leach solution is at least 10%greater than the concentration of the metal in the second clarifiedaqueous leach solution; (e) subjecting the diluted second portion to asecond solvent extraction whereby a second aqueous raffinate isobtained; wherein the aqueous stream for diluting the second aqueousleach pulp comprises at least a portion of the second aqueous raffinate;and (f) recycling at least a portion of the first aqueous raffinate andat least a portion of the second aqueous raffinate to the leachingprocess.
 22. The process according to claim 21, wherein theconcentration of the metal in the first clarified aqueous leach solutionis at least 50% greater than the concentration of the metal in thesecond clarified aqueous leach solution.
 23. The process according toclaim 21, wherein the concentration of the metal in the first clarifiedaqueous leach solution is at least 100% greater than the concentrationof the metal in the second clarified aqueous leach solution.